Threaded connection of two metal tubes with groove in the threading

ABSTRACT

A threaded connection for two metal pipes including a male element at the end of a first pipe and a female element at the end of a second pipe, the male element including a male external thread, the female element including a female internal thread. The male and female threads are screwed one into the other. At least one of the male or female threads includes a groove formed in the thread and opening either into a stabbing flank or into a crest of the thread or straddling the stabbing flank and the thread crest. The groove is disposed in the thread so as to increase the flexibility of the portion of the thread which comes into pressure contact with the mating thread or to reduce the contact surfaces or to affect the two aspects at the same time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Such connections are known, in particular for strings of casing pipes orproduction tubing or strings of drill pipes for hydrocarbon wells.

2. Discussion of the Background

In the remainder of the present document, the term “threaded connectionfor two metal pipes” will encompass both a so-called integral connectionbetween two long pipes and a connection between a first long pipe and asecond, short, pipe such as a coupling.

Thus the American Petroleum Institute (API) defines:

in specification API 5CT, metal pipes and threaded metal pipeconnections for production and for casing hydrocarbon wells;

and in specification API 5B, standard tapered thread forms for suchconnections and standard thread forms for triangular, “round” ortrapezoidal threads.

API 5B triangular or round thread forms comprise, on each of the maleand female elements, two lateral flanks—a load flank and a stabbingflank—each at 30° either side of a plane normal to the axis of theconnection.

At the end of the connection makeup, because of the taper of the threadforms, each of the two flanks is in contact under a metal-metal contactpressure with its mating flanks and a clearance exists between the crestand root of the mated threads, the crests and roots of the threads beingvery rounded in the case of round threads.

API 5B trapezoidal thread forms comprise, on each of the male and femaleelements, two lateral flanks—a load flank and a stabbing flank—slightlyinclined with respect to a plane normal to the connection axis, a threadroot and a thread crest generally parallel to the thread taper, theshape of the female threads mating perfectly with that of the malethreads.

At the end of makeup of this type of connection with API 5B trapezoidalthreads, because of the taper of the thread forms, the thread crest ofat least one of the male or female elements is in contact, under ametal-metal contact pressure, with the root of the thread of the matedelement; further, the load flanks are also in contact while a clearanceexists between the stabbing flanks, at least in the most frequentlyencountered case where the elements of the connection are under tensionfrom the weight of the string, for example, or from the bearing reactionof the abutments.

Such connections with an axial clearance are termed “with interferingthreads” since they develop a radial interference between the matingthreaded surfaces which results in a metal-metal contact pressurebetween these surfaces and in an increase in the makeup torque, theincrease being due to the friction to be overcome. The intensity of theradial interference is measured by the value of the diametricalinterference.

As is known, the term “diametrical interference” means the difference indiameter between a point on the male element and the mating point on thefemale element when the elements are not connected; a positive value forthe diametrical interference means that a contact pressure exists in thecontact zones of the connection; a zero value for the diametricalinterference means a simple contact without a contact pressure, and anegative value for the diametrical interference means a radialclearance.

In other types of connections such as that described in European patentapplication EP-A-0 454 147, the thread form is designed so that the twolateral flanks just come into contact at the end of connection makeup;in contrast a clearance can be provided between the roots and crests ofmated threads.

Such connections, termed “trapezoidal threaded with no axial clearance”in the remainder of the document, allows operation both in axialcompression and in axial tension or bending.

Other connections such as those described in U.S. Pat. Nos. 2,211,179,4,161,332 or 4,537,428 use a two-step straight thread with trapezoidalthreads. Such thread forms normally do not have radial interference,contact between the threads only occurring at the load flanks, inparticular the load flanks when the connection is subjected to axialtension forces.

US-RE30647 describes a variation of a threaded connection with two-stepstraight threads and a trapezoidal thread form known as wedge threads,the width of the male and female threads varying along the length ofeach step of the thread in a co-ordinated manner such that, as the maleelement is gradually screwed into the female element, the axial spacesbetween the male and female flanks existing on engaging the elementsreduce to zero; the lateral flanks thus act as an abutment and toposition the independent sealing surfaces.

In the remainder of the present document, the term “wedge thread” or“variable width thread” will be used for a thread with suchcharacteristics.

US RE34467 describes an adaptation of a variable width thread on aconnection comprising tapered interfering threads, with the aim ofavoiding overpressures of grease between the roots and crests of thethreads, which overpressures can cause erroneous measurements for themakeup torque in connections of the type described in US RE30647.

International patent application WO 94/29627 describes an adaptation ofa variable width thread on a connection comprising tapered interferingthreads with the aim of making up the connection to a very high makeuptorque. In WO 94/29627, making up of a wedge thread is continued afterthe male lateral flanks of the thread have come into contact with thefemale flanks, the total surface of the thread flanks enabling a veryhigh makeup torque to be exerted with no risk of plastification.

In the majority of threaded connections, in particular those withtapered threads, the makeup torque increases regularly during makeupand/or increases suddenly at a given moment but in any event it mustremain below a value corresponding to that for plastification of themetal, as plastification causes permanent deformation of the metal andthere is a risk of galling in the plasticized zones after severalmakeup-breakout operations.

If the slope of the graph of makeup torque vs number of screw turns ishigh, the maximum allowed torque will be reached after a relatively lowrotation of one element relative to the other: this is the case, forexample, in a threaded connection with threads which interfere verystrongly or after the lateral flanks on a connection with trapezoidalthreads with no axial clearance or with variable width threads come intocontact.

Since machining tolerances cause variable slopes in the torque curvesdepending on matching of the male/female threads, it is not possibleunder such conditions to predict the final relative position of theelements of the connection for the maximum allowed value of the torque.

The above disadvantages are amplified when each male and female elementalso comprises a metal-metal sealing surface and an abutment such as,for example, in EP-A-0 454 147 as the action of the thread, the sealingsurface and the abutment at the end of screwing must be synchronised atthe end of connection makeup for all male-female element matingscenarios.

SUMMARY OF THE INVENTION

The present invention seeks to provide a connection between two metalpipes using tapered, straight or straight-tapered threads with differentthread forms and connection clearances which can overcome thedisadvantages described above.

The term “thread” as used in the remainder of the document means thetotality of the threaded portions of an element. A thread can thus beconstituted by a single threaded portion or by a plurality of threadedportions, for example two stepped straight threaded portions or by onetapered threaded portion and one cylindrical threaded portion of thetype described in U.S. Pat. No. 5,437,429, the thread of that patentbeing termed straight-tapered.

In accordance with the invention, a threaded connection for two metalpipes comprising a male element at the end of a first pipe and a femaleelement at the end of a second pipe, the male element comprising a maleexternal thread, the female element comprising a female internal threadwherein each of the thread flanks is parallel to the corresponding flankon the male thread, the male and female threads being screwed one intothe other, is such that at least one of the male or female threadscomprises a groove formed in the thread and opening either into thestabbing flank or into the crest of the thread or straddling thestabbing flank and the thread crest.

The fact that each of the flanks of the female thread is parallel to thecorresponding male flank enables the elements to be connected withoutone or other of the flanks bearing only at a point on the mated flank.

The term “groove” means a cavity with two groove walls and a groovebase, the developed length in the thread being long with respect to itswidth and to its depth, the latter dimensions being measured in a crosssectional plane passing through the pipe axis.

The groove profile corresponds to the intersection of the groove by itscross section and the width of the groove is measured at this profile ata given depth.

Until now, threads have been considered to be a solid whole and pastattempts have been to reinforce them. It is thus surprising that we havenoticed the importance of considering them to be a structure thestiffness of which can be modified by forming a groove therein.

The function of the groove formed in the thread of the present inventionis to reduce contact forces between the male and female threads and thusin particular to reduce the makeup torque which is proportional to thecontact forces.

To this end, the groove can be disposed in the thread so as either toincrease the flexibility of the portion of the thread which comes intocontact under pressure with the mating thread, or to reduce the contactsurfaces, or to affect these two aspects at the same time.

Examples of dispositions for the groove satisfying these functions onthe threaded connections of the invention will be described below fordifferent types of threads and for different thread forms.

U.S. Pat No. 3,882,917 and French patent FR-A-2 408 061 describethreaded connections in which one of the threads possesses a kind ofgroove opening at the thread crest but such grooves are strictlyassociated with thread flank structures to obtain a self-blockingconnection, i.e., resisting breakout.

In the case of U.S. Pat. No. 3,882,917, one of the thread flanks has aprotruding rib with three faces one face of which bears against thecorresponding flank of the mating thread, the two other faces delimitinga type of groove which enables the rib to bend.

In FR-A-2 408 061, applicable to trapezoidal threads, the inclination ofthe flanks of the grooved thread is different from that of the nongrooved thread and is such that the width of the groove at its openingreduces during makeup under bending forces resulting from the differencein the orientation of the flanks between the mated threads.

None of those documents discloses the function of the groove in theconnection in accordance with the invention and none applies to femalethreads where each of the thread flanks is parallel to the correspondingflank of the male threads.

The groove of the invention can thus be formed in tapered, straight orstraight-tapered threads, with single or a plurality of steps,interfering threads or threads with no axial clearances and with aconstant thread width or with a thread width which is variable along thethread.

The groove can be formed in triangular, round or trapezoidal threads;the term “trapezoidal threads” as used in the remainder of the presentdocument including threads with a negative load flank angle of the typedescribed in EP-A-0 454 147 or with a positive load flank angle and withdovetail shaped threads of the type described in US RE 30647, or halfdovetail, as described in WO 94/29627.

The groove can be formed over all or a portion of the male or femalethread or in both simultaneously.

It can also be formed alternately on portions of the male and femalethread.

It can also be continuous or discontinuous on the male or femalethreads.

For threads comprising runout threads, the groove may be formed only inthreads termed perfect threads, i.e., of full depth, or it may be alsoformed in imperfect threads.

A wide variety of groove profiles can be used, for example asemi-circular groove, a U with parallel or non parallel branches, asymmetrical or asymmetrical V or a combination of these shapes, inparticular a U or a V with a rounded base with a given radius, or a morecomplex and non symmetrical profile.

The groove profile is preferably constant over all of its length.

When the groove opens into the crest of the thread, the axis of thegroove profile may be perpendicular to the axis of the connection orinclined to that perpendicular, depending on the case.

When the groove opens into the stabbing flank, the axis of the grooveprofile may be parallel to the axis of the connection or inclined to it,depending on the case.

Optionally, the depth or width of the groove, or both, may vary over itslength.

The base of the groove preferably has a radius of 0.2 mm or more tolimit stress concentration at that location.

Preferably, when the groove opens into the crest of the thread, thewidth of the groove measured at its opening is less than or equal to ⅔of the width of the thread.

In the remainder of the present document, the term “thread width” meansthe width measured axially at the half-height of the thread, and theterm “thread depth” means the distance measured in a plane perpendicularto the axis of the connection between the root and crest lines of thethread.

Preferably, when the groove opens into the stabbing flank, the groovewidth at its opening is less than or equal to ⅔ of the thread depth.

Preferably again, when the groove is located on the stabbing flank ofthe thread, its depth is less than or equal to ⅔ of the thread depth.

Preferably again, when the groove is located on the thread crest, itsdepth is less than or equal to the thread depth such that the base ofthe groove does not go beyond the line joining the thread roots.

Preferably again, when the groove straddles the thread crest andstabbing flank, its opening width and its depth satisfy both thecriteria for a groove opening into the thread crest and for a grooveopening into the stabbing flank. For this c reason, its depth is lessthan or equal to the lesser value of the thread depth and ⅔ of thethread width and its opening width is less than or equal to ⅔ of thelesser value of the thread width and the thread depth.

Advantageously, the male and female elements of the threaded connectionof the invention each comprise at least one metal-metal sealing surface,each male sealing surface located on the male element radiallyinterfering with a female surface located in a corresponding manner onthe female element so as to create at least one metal-metal sealingcontact between the male and female elements at the end of theconnection makeup.

Advantageously again, the male and female elements of the threadedconnection of the invention each comprise an abutment, the male abutmentlocated on the male element bearing on the female abutment located onthe female element to precisely determine the position of completeconnection makeup and placing the load flanks of the male and femalethreads under contact pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

Different non limiting embodiments of threaded connections of theinvention will now be described.

FIG. 1. is a diagram of a prior art threaded and coupled (T & C)connection for two pipes with tapered threads.

FIG. 2 is a diagram of a variation of the same type of prior artconnection.

FIG. 3 is a diagram of a prior art integral connection for two pipesusing a two-step sight thread.

FIG. 4 is a diagram of a detail of a thread with a round thread formwith a groove, in accordance with the invention.

FIGS. 5 to 7 are diagrams of a detail of a thread with a trapezoidalinterfering thread form with a groove in accordance with the invention,each figure representing a different disposition for the groove.

FIGS. 8 to 10 are also diagrams of a detail of a thread with atrapezoidal thread form with no axial clearance with a groove inaccordance with the invention, each figure representing a differentdisposition for the groove.

Each of FIGS. 4 to 10 is composed of three sub-figures, suffix Acorresponding to the female thread, suffix B to the male and suffix C tothe two connected together.

FIG. 11 shows in more detail the disposition of the groove on a threadof the type shown in FIG. 8, FIG. 11A relating to the female thread andFIG. 11B to the male thread.

FIG. 12 is a diagram of a detail of a straight thread with edge threadsof variable width and a groove in accordance with the invention, FIGS.A, B,C and D being respectively relevant to the female thread, the malethread, to the connection during makeup and to the made up connection.

FIG. 13 is a schematic graph of the makeup torqueT against the number ofturns N for a pipe connection with a thread in accordance with FIG. 12.

FIG. 14 is a diagram of a detail of a tapered thread with wedge threadswith a variable width and a groove in accordance with the invention,FIGS. A, B, C and D respectively relating to the female thread, the malethread, to the connection during makeup and to the made up connection.

All of the figures are simply diagrams; the tapers and clearances inparticular are not drawn to scale and have been magnified to enablebetter comprehension of the operation of the connections.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a threaded connection 100 in accordance with specificationAPI 5CT between two metal pipes 101 and 101′ using a coupling 102.

Each end of pipe 101, 101′ comprises a male element comprising a maletapered thread 103, 103′ with round threads.

Coupling 102 comprises two female elements disposed symmetrically withrespect to the medial plane of the coupling, each female elementcomprising a female tapered thread 104, 104′ with threads which matewith those of the male element.

Male threads 103, 103′ are screwed into mating female threads 104, 104′.

For this type of connection, specification API 5B defines the threadform, their dimensions, the thread taper, the thread pitch, etc. . .

Although it has not been shown, a “Buttress” type connection can also beused in accordance with specifications API 5CT and 5B, similar to thatof FIG. 1 but with trapezoidal threads.

FIG. 2 shows a known T & C connection 200 with tapered threads 203, 204and with a trapezoidal thread form, coupling 202 having a lug 206 in itscentral portion, which lug renders the flow of fluid in pipes 201, 201 ′non turbulent and can house female abutments 210 which bear against maleabutments 209 constituted by the annular end surfaces of the pipes.

Tapered male 207 and female 208 sealing surfaces located on the nonthreaded portions and radially interfering so as to form an elasticcontact pressure between them and enable the sealing of the connectionof FIG. 2 in known manner.

The tapered shape of male and female abutments 209, 210 also reinforcethe contact pressure at the sealing surfaces 207, 208 and thus increasethe seal of the connection.

FIG. 3 shows an integral threaded connection between two pipes 301 and302 comprising two-step straight threads.

The end of pipe 301 comprises a male element comprising a straighttwo-step male thread 303, 303′, a male tapered shoulder surface 307 in ahalf dovetail form between the two male threaded steps and abutments309, 309′ at each end of the male element.

The end of pipe 302 comprises a female element which mates with the maleelement and comprises a female straight two-step thread 304, 304′, afemale tapered shoulder surface 308 in a half dovetail form between thetwo female threaded steps and abutments 310, 310′ at each end of thefemale element.

The male and female threads of connection 300 have a trapezoidal threadform and normally do not radially interfere after screwing.

In the made up state, shoulders 307, 308 form the principal abutment,abutments 309, 309′, 310, 310′ only acting as a safety abutment in theevent of burying the principal abutment.

Close to the end of the male element, the tapered surfaces 311′, 312′ onthe male and female elements respectively form an internal pair ofmetal-metal sealing surfaces. Close to the end of the female element,the tapered surfaces 311, 312 form an external pair of metal-metalsealing surfaces. The external pair 311, 312 of sealing surfaces couldalso be located between shoulders 307, 308 and the large diameter threadstep 303, 304.

FIGS. 4 to 10 and 12, 14 schematically illustrate the operation of agroove of the invention on threaded connections of the type shown inFIGS. 1 to 3. For clarity, FIGS. 4 to 10 and 12, 14 show only a few maleand female threads.

FIG. 4 shows portions of tapered threads with round thread forms with agroove on a connection of the type shown in FIG. 1, in accordance withthe invention.

Male threads 11 (FIG. 4B) comprise a load flank 13 and a stabbing flank15 each at 30° either side of a plane perpendicular to the connectionaxis while the thread root 17 is rounded.

A groove 21 is cut radially into the crest of the male thread with a Vprofile with a rounded base.

The width of groove 21 at its opening is of the order of 25% of thewidth of thread 11, its depth being equal to 50% of the thread depth.

The radius at the base of the groove is 0.4 mm.

Female threads 12 (FIG. 4A) comprise a load flank 14 and a stabbingflank 16 mating with those of the male threads, the thread root beingrounded.

The load flank 14 and stabbing flank 16 are each located at 30° to eachside of a plane perpendicular to the connection axis and are thusrespectively parallel to male flanks 13, 15.

A groove 22, shown as a dotted line and identical in shape anddisposition to groove 21, can be radially cut into the crest of thefemale thread in place of or complementary to the groove 21 of the malethread.

Grooves 21 and/or 22 at the thread crest increase the flexibility of thethreads and thus reduce the contact pressure on the lateral flanksthereof and reduce the makeup torque compared with solid threads.

FIG. 5 shows portions of tapered threads with an interfering trapezoidalthread form with a groove 41 opening into the stabbing flank, inaccordance with the invention, on a connection of the type shown in FIG.2.

Male threads 31 (FIG. 5B) comprise a load flank 33 the orientation ofwhich is substantially normal to the connection axis, and a stabbingflank 35 inclined at about 10° to the plane normal to this axis, athread root 37 and a thread crest 39.

The thread roots 37 and thread crests 39 are parallel to the taper ofthe thread.

Groove 41 partially separates the male thread 31 into two portions; ithas a V profile with a rounded base orientated substantially parallel tothe connection axis.

The width of groove 41 at its opening into the stabbing flank is ⅓ ofthe thread depth and its depth is 50% of the thread width.

The vertex angle of the V of groove 41 is about 30° and the branch 42 ofthe V on the root side of the male thread is substantially parallel tothe connection axis.

The groove base is an arc with a radius of 0.4 mm.

The form of female threads 32 (FIG. 5A) mates with that of thecorresponding male threads 31, i.e., their contours are parallel to thecontours of the corresponding male threads, in particular the loadflanks 33, 34 are parallel to each other and the stabbing flanks 35, 36as well; the depth of the female thread is, however, slightly less thanthat of the male thread and the root width of the female thread 40 isslightly larger than that of the mating male crests 39.

The female threads 32 do not comprise a groove.

FIG. 5C schematically illustrates the operation of the connection at theend of the makeup.

At the end of the connection makeup, the mating load flanks 33 and 34are in contact under pressure because, for example, of the reaction ofthe abutments or of an axial tensile force on the connection. Thiscontact pressure is distributed over the whole width of the load flanksbecause the male load flanks are parallel to the female load flanks.

Similarly, the crests 39 of the male threads are in radially interferingcontact with the roots 40 of the female threads. In contrast, because ofthe geometry described above, a clearance is provided between the matingstabbing flanks 35 and 36 and between the male thread roots 37 and thefemale thread crests 38.

The upper portion 43 of male thread 31, subjected to the radialinterference of surfaces 39 and 40, can bend elastically and reduce thecontact pressure and thus the makeup torque.

The flexibility of the upper portion 43 of male thread 31 can beadjusted by adjusting the position of groove 41 on the stabbing flank 35and by adjusting the depth of the groove.

Because of the absence of radial interference between the male threadroot 37 and the female thread crest 38, the presence of a groove openinginto the stabbing flank 36 of the female thread is not operationallyjustified.

Such a disposition of the groove on the interfering threads can inparticular be of interest as regards reducing the contact pressurebetween the thread crests and roots at the level of the threads close tothe metal-metal sealing surfaces; a high contact pressure here induces adischarge thereof between the sealing surfaces: thus a groove is onlyprovided in the portion of the thread close to the bearing surface.

Although it has not been illustrated, it is also possible in asymmetrical way to envisage a connection with radial interferencebetween the roots of male thread 37 and the crest of the female thread38 and a groove formed in the female thread and opening onto thestabbing flank 36.

FIG. 6 shows portions of tapered threads with interfering trapezoidalthread form in accordance with the invention, similar to those of FIG. 5but groove 51, with a V profile with a rounded base, is formed in themale threads so as to open into thread crest 39.

The function of groove 51 is firstly to reduce the radially interferingsurface areas 39 and 40 so as to reduce the frictional force which isproportional to surface areas and secondly to increase the flexibilityof the bearing portion 53 of the thread when the load flanks 33 and 34are under contact pressure, which also reduces frictional forces, andthus the makeup torque of the connection.

The shape of groove 51 is similar to that of groove 41, but it islocated radially and opens into the thread crest 39, towards stabbingflank 35. Its opening width is thus, for example, 60% of the width ofthe male thread and its depth is 50% of the thread depth.

As in FIG. 5, the presence of a groove at the female thread crest 38 isnot justified, as no friction is produced here with the male thread root37.

FIG. 7 shows portions of tapered threads with an interfering trapezoidalthread form similar to those of FIG. 5, except that the groove 56 with aV profile and a rounded base is disposed in the male threads so as toopen straddling male thread crest 39 and male stabbing flank 35.

Because of its straddling position, the orientation of groove 56 in thebulk of the thread is oblique and the axis of the groove profile formsan angle of 45°, for example, to the thread crest 39.

The disposition of groove 56 can both increase the flexibility of thethread, the faces 33 and 39 of which are subjected to contact pressures,and reduce the contact surfaces 39, 40.

FIG. 8 shows portions of tapered threads with a trapezoidal thread formwith no axial clearance in accordance with the invention, a groove 61being formed in the crest 39 of the male thread 31 and a further groove62 being formed in the crest 38 of female thread 32.

The form of threads 31, 32 is similar to that of the threads in FIG. 5;in particular the contour of the female threads 32 is parallel to thecontours of the corresponding male threads 31, except that the crests 39of male thread 31 and the roots 40 of female thread 32, the roots 37 ofthe male thread and the crests 38 of the female thread are disposed soas not to interfere radially during makeup while lateral flanks 33, 34,35 and 36 are disposed such that they just come into contact at the endof the connection makeup, as shown in FIG. 8C.

In Common with the preceding figures, the male 61 and female 62 grooveshave a V profile with a rounded base.

In common with the preceding figures, the vertex angle of the V ofgrooves 61, 62 is about 30° and the radius of the point of the V is 0.4mm.

The depth of grooves 61, 62 is, for example, 70% of the thread depth andtheir opening width in the thread crest is 35% of the thread width.

Grooves 61, 62 act to cut half-teeth 63, 65 into the male thread and 64and 66 into the female thread.

These half-teeth are far more flexible than a solid tooth when themating lateral flanks 33, 34, 35, 36 come into contact at the end of theconnection makeup. Thanks to this increased flexibility, the action ofabutments such as 209, 210 in FIG. 2 can be synchronised, with theapproach of the mating lateral flanks; premature approach of theseflanks because of poorly matched male and female elements, which occursin a small proportion of manufacturing means, in this case simplyresults in a slight elastic deformation of the threads at the end ofconnection makeup but correct positioning of the abutments 209, 210,necessary to obtain satisfactory sealing of the sealing surfaces 207,208, is obtained.

The following example shows an application of such a configuration:

external pipe diameter: 177.8 mm (7″);

pipe thickness: 10.36 mm (29 lb/ft);

material treated for a minimum yield stress of 551 MPa;

tapered thread with 5 trapezoidal threads per inch with a 6.25% taper(i.e., an angle of 1.79° between the pitch tapers 45, 46 and thedirection 47 of the connection axis).

FIGS. 11B and 11A show a detail of FIG. 8 with the respective form ofmale and female threads 31 and 32 which are dimensioned so as not tohave any axial clearance after makeup.

In the Figures, load flanks 33, 34 are at slightly negative angles of−3° while stabbing flanks 35, 36 are both inclined at 13° with respectto the normal to the connection axis. The crests 38, 39 and roots 37, 40of the threads are parallel to the generatrices of the pitch tapers 45,46 which are inclined at 1.79° to the connection axis.

The thread width is close to 2.54 mm and the thread depth is 1.6 mm.

The axis of grooves 61, 62 disposed respectively on the male thread 31and on the female thread 32 is perpendicular to the connection axis andthey open into the thread crest 39, 38.

Grooves 61, 62 have two inclined groove walls 71, 71′, 72, 72′ andconnected by a groove base 73, 74 such that the groove profile is anasymmetrical V shape with a rounded base.

The distance between the axis of grooves 61, 62 and the correspondingload flank 33, 34 in this case is 1.4 mm.

The vertex angle of the V is 35°, wall 71, 72 directed towards the loadflank 33, 34 being a little more inclined to the V profile axis than theother wall 71′, 72′.

The radius at the base of groove 73, 74 is 0.4 mm.

The depth of grooves 61, 62 in FIG. 11 is 1 mm, namely 62.5% of thethread depth. The opening width is thus 38% of the thread width.

Although not shown, a threaded connection with trapezoidal threads ofthe invention can also be produced in which the tapered threads do nothave an axial clearance but interfere radially when made up, at leastone of the threads having a groove opening into the thread crest.

FIG. 9 of the invention shows portions of tapered threads withtrapezoidal threads and an axial clearance as is the case for FIG. 8 buta groove 67 has been produced in male thread 31 opening into itsstabbing flank 35 to reduce the contacting frictional surfaces.

Groove 67 has an axis which is substantially close to the connectionaxis.

As an example, its depth, measured axially, can be 50% of the width ofthe thread and its width at its opening is 50% of the depth of the malethread.

Its profile, as in the preceding figures, is a V with an angle of about30° with its point ending in a radius of 0.4 mm.

The presence of a groove in the female stabbing flank in addition togroove 67 is not justified.

FIG. 10 shows a groove 68 opening so as to straddle the crest of malethread 39 and the male stabbing flank 35 of the connection in accordancewith the invention with tapered threads with a trapezoidal thread formwith no axial clearance of the type shown in FIG. 8.

As was the case for FIG. 7, the groove is inclined with respect to thethread crest and the stabbing flank and can reduce both the contactpressure on the lateral mating flanks and the frictional contactingsurfaces.

Although it has not been shown, it is possible in accordance with theinvention to produce a connection with straight threads and atrapezoidal thread form with a groove, the groove or grooves beinglocated, for example, at the thread crest or straddling the thread crestand the stabbing flank and intended to reduce the contact pressure onthe load flanks.

It is also possible, in accordance with the invention, to envisage agroove at the crest thread on straight, tapered or straight-taperedtrapezoidal wedge threads where the width of the male and female threadvaries along the length of the thread, so that the lateral flanks of thethreads are brought into a contact pressure at the end of makeup.

FIG. 12 illustrates a groove disposed at the crest of a variable widthwedge thread on threads 303′, 304′ of a two-step straight threadconnection of the type shown in FIG. 3.

Male threads 331 of FIG. 12B are trapezoidal and comprise a load flank333, a stabbing flank 335, a thread crest 339 and a thread root 337, thecrests and roots being located on cylindrical surfaces coaxial with theconnection axis.

The load and stabbing flanks form, with the normal to the connectionaxis, an angle which conventionally is taken to be negative such thatthread 331 is in the form of a dovetail.

The width l 1 of the male thread continuously increases on each stepfrom the threads located at the free end side of the male elementtowards those located on the opposite side.

The female elements 332 of FIG. 12A are trapezoidal and dovetailed matedwith that of the male threads 331 and they comprise a load flank 334, astabbing flank 336, a thread crest 338 and a thread root 340, the crestsand roots of the thread being located on cylindrical surfaces coaxialwith the connection axis.

Each of the female thread flanks 334, 336 is parallel to the respectivecorresponding flank 333, 335 of the male thread.

The width l 2 of the female threads continuously increases on the samestep from the threads located at the free end side of the female elementtowards those located on the opposite side, in a manner whichco-ordinate with the evolution of the width of the male threads on themale thread.

The female threads comprise a groove 362 which opens into the threadcrest 338 the form and disposition of which are similar to 62 in FIG.8A.

FIG. 12C shows the male and female threads of a step during makeup: aconstant space x exists between load flanks 333, 334 of each threadengaged with its mate.

Further, with this type of straight thread, there is no radialinterference between the mated crests and roots 337-338, 339-340.

As the male element is screwed into the female element, the spacebetween the mating lateral flanks reduces to zero as shown in FIG. 12D.

If screwing is continued beyond this point, the male lateral flanks 333,335 press into the mating female flanks 334, 336, the contact pressurebetween the mating flanks increasing as screwing is continued. Thisresults in a very rapid rise in the makeup torque and, depending on thedeveloped surface of the flanks, makeup can be completed with under anextremely high torque.

Groove 362, like in the preceding figures, has a V profile with arounded base, the vertex angle of the V of the grooves being about 30°and the radius of the point of the V being 0.4 mm. Their depth, forexample, is 70% of the thread depth and their width at the opening intothe thread crest is 35% of the thread width.

Groove 362 cuts half-teeth 364 and 366 into female thread 332, whichhalf teeth are much more flexible than a solid tooth.

The presence of a groove in such threads 331, 332 with a variable widthresults in a reduction in the slope of the makeup torque T—elementrotation N curve as shown by the appearance of the curve E in FIG. 13with respect to curve D relating to similar threads with no groovealthough the maximal admissible makeup torque is slightly reduced by thepresence of the groove.

The presence of a groove in such threads with a variable width alsoresults in a self-limiting effect of the torque T beyond a certainthreshold. This prolongs the rotation N possible before achieving themaximum admissible makeup torque and enables one or more pairs ofseparate sealing surfaces 307, 308 to be correctly positioned using oneor more pairs of abutments 309, 309′, 310, 310′ in all male-femalemating scenarios.

FIG. 14 shows a groove located at the crest of a wedge thread with avariable width on threads 203, 204 of a tapered thread connection of thetype shown in FIG. 2.

Male threads 231 in FIG. 14B are trapezoidal in form and comprise a loadflank 233, a stabbing flank 235, a thread crest 239 and a thread root237, the crests and roots of the thread being disposed on the taperedsurfaces coaxial with the connection axis and with the same taper.

In an alternative (not shown), the crests and/or roots of the threadscan be parallel to the connection axis, the pitch surface neverthelessbeing tapered.

The load flanks 233 form, with the normal to the axis of the connection,an angle which is conventionally taken to be negative while the stabbingflanks 235 form, with the normal to the axis of the connection, an anglewhich is conventionally taken to be positive.

The angle between the load flank and the stabbing flank of a thread issuch that the flanks 233, 235 diverge on travelling towards the threadcrest 239 such that thread 331 is in a form known as a half dovetail.

The width of the male thread continuously increases from the threadslocated on the side of the free end of the male element towards thoselocated on the opposite side.

The male threads comprise a groove 261 which opens into the threadcrests 239 the form and disposition of which are similar to 61 in FIG.8B.

The female threads 232 of FIG. 14A are trapezoidal mated with those ofmale threads 231 and they comprise a load flank 234, a stabbing flank236, a thread crest 238 and a thread root 240.

Each of flanks 234, 236 of the female thread is parallel to therespective corresponding flank 233, 235 of the male thread.

The width of the female threads increases continuously from the threadslocated on the side of the free end of the female element towards thoselocated on the opposite side, in a manner coordinated with the evolutionof the width of the male threads of the male thread form.

FIG. 14C shows male and female threads during makeup: a constant space xcan be seen between the load flanks 233, 234 of each thread taken withits mate.

As the male element is screwed into the female element, space x betweenthe mating lateral flanks reduces to zero as shown in FIG. 14D. Ifmakeup is continued beyond this point, the male lateral flanks 233, 235press into the mating female flanks 234, 236, the contact pressurebetween the mating flanks increasing as makeup is continued.

This results in a very rapid increase in the makeup torque and,depending on the developed surface of the flanks, makeup can becompleted under a very high torque.

Further, with this type of wedge thread tapered thread, it can bearranged such that when makeup is complete, there is a radialinterference between the crests and roots of the mated thread 237-238and/or 239-240 such that male threads 231 completely fill the hollowsbetween the female threads and/or the female threads 232 completely fillthe hollows between the male threads. FIG. 14D shows a case of completefilling of the male and female threads, the thread thus being tight initself and enabling the presence of grease between the crests and rootsof the mated threads on completion of makeup to be avoided, whichpresence is a cause of an erroneous measure of the makeup torque.Preferably, it is arranged such that a radial interference developsbefore the mating lateral flanks come into contact.

As in the preceding figures, male groove 261 has a V profile with arounded base, the vertex angle of the V of the grooves being about 30°and the radius of the point of the V being 0.4 mm. Its depth is, forexample, 70% of the thread depth and its width at its opening into thethread crest is 35% of the thread width.

The groove cuts half teeth 263, 265 into male thread 231, which halfteeth are much more flexible than a solid tooth and fulfil the samefunctions as the groove 362 in the connection of FIG. 12, whichfunctions have been described above in the text. It can also bettersynchronise the axial extension of the lateral flanks with the radialinterference of the crests and roots towards the end of makeup.

The connection of the present invention can be produced in a number ofvariations, the several examples of which given here not in any waybeing limiting in nature.

In particular, the present invention can be applied to:

an integral threaded connection as shown in FIG. 3, a male element beingdisposed at the end of a first long metal pipe and a female elementbeing disposed at the end of a second long metal pipe,

and to a T & C connection as shown in FIGS. 1 and 2, in which two longmetal pipes comprising a male element at its end are connected via ametal coupling each end of which is provided with a female element, sucha T & C connection constituting two threaded connections of theinvention.

What is claimed is:
 1. A threaded connection for two metal pipescomprising a male element at the end of a first pipe and a femaleelement at the end of a second pipe, the male element comprising a maleexternal thread, the female element comprising a female internal thread,wherein each thread flank is parallel to a corresponding flank of themale thread, the male and female threads being screwed one onto theother, characterized in that at least one of the male or female threadscomprises, with the aim of reducing the contact forces between the maleand female threads, a groove comprising two groove walls and a groovebase, which is provided in the bulk of the thread and which opens eitherinto a stabbing flank or into a thread crest or straddling the stabbingflank and the thread crest.
 2. A threaded connection according to claim1, characterized in that the groove is continuous over a developedlength of the thread or threads.
 3. A threaded connection according toclaim 1, characterized in that the groove has walls with a V profile anda rounded base.
 4. A threaded connection according to claim 1,characterized in that the base of the groove has a radius of 0.2 mm ormore.
 5. A threaded connection according to claim 1, characterized inthat the width of the opening of the groove opening into the threadcrest is less than or equal to ⅔ of the thread width.
 6. A threadedconnection according to claim 1, characterized in that the width of theopening of the groove opening into the stabbing flank is less than orequal to ⅔ of the thread depth.
 7. A threaded connection according toclaim 1, characterized in that the width of the opening of the groovestraddling the thread crest and the stabbing flank is less than or equalto ⅔ of the lesser of the width or depth of the thread.
 8. A threadedconnection according to claim 1, characterized in that the depth of thegroove, located on the thread crests is less than or equal to the threaddepth.
 9. A threaded connection according to claim 1, characterized inthat the depth of the groove formed in the stabbing flank is less thanor equal to ⅔ of the thread width.
 10. A threaded connection accordingto claim 1, characterized in that the depth of the groove straddling thethread crest and the stabbing flank is less than or equal to the lesserof the thread depth and ⅔ of the thread width.
 11. A threaded connectionaccording to claim 1, characterized in that the male and female threadsare tapered with trapezoidal threads disposed in one step or in aplurality of steps.
 12. A threaded connection according to claim 11,characterized in that at least one of the male and female thread crestsradially interferes with the mated thread root or roots when makeup iscomplete.
 13. A threaded connection according to claim 11, characterizedin that the male and female threads are trapezoidal with no axialclearance.
 14. A threaded connection according to claim 1, characterizedin that the male and female threads are straight, disposed in one or aplurality of steps, or are straight-tapered.
 15. A threaded connectionaccording to claim 1, characterized in that the width of the male andfemale thread varies along the entire length of the thread or of eachthreaded portion in a co-ordinated manner to constitute wedge threads.16. A threaded connection according to claim 1, characterized in thatthe male and female threads are tapered with round threads.
 17. Athreaded connection according to claim 1, characterized in that the maleelement comprises at least one male sealing surface and the femaleelement comprises at least one female sealing surface, the correspondingmale and female sealing surfaces radially interfering when makeup iscomplete so as to create at least one metal-metal sealing contactbetween the male and female elements.
 18. A threaded connectionaccording to claim 1, characterized in that the male and female elementeach comprise at least one abutment, the abutment or abutments of themale element bearing against the corresponding abutments of the femaleelement at the end of the connection makeup.
 19. A threaded connectionaccording to any claim 1, characterized in that it is an integral type.20. A threaded connection according to claim 1, characterized in that itis of a threaded and coupled type.
 21. A threaded connection accordingto claim 14, characterized in that the width of the male and femalethread varies along the entire length of the thread or of each threadedportion in a co-ordinated manner to constitute wedge threads.